Microfluidic chips, such as labs on chip (LOCs), including micro-Total Analysis Systems (IJTAS), are increasingly being used for small volume sample testing in a wide variety of fields, such as medicine, pharmaceutical research, food and water analysis, pathogen detection, etc. A great number of processes (filtration, thermal processing, mixing, loading and rinsing, reacting, SPR, PCR, etc.) have been demonstrated on a variety of substrates for a variety of test materials. Remarkable results can be obtained with the precise control and manipulation of volumes of liquids of only a few microliters.
A droplet of an aqueous solution/suspension (the most common liquid used in microfluidics) or an oil, will exhibit surface tension that results in beading. Herein liquid refers to a liquid or a liquid that suspends, contains or surrounds solids or gasses, be it as a suspension, solution, colloid, dispersion, or with less regularity, e.g. as beads in a stream. Surface tension of the liquid tends to lead to beading, which can make it difficult to control movement of the microfluid, as it may stay in a part of a chamber and not approach a desired exit, and the separation of beads leads to uncoordinated movement of the fluid. Generally the force of gravity is, in itself, insufficient to draw fluid through microfluidic channels, and air pressure differences have a much greater effect.
It is known to provide walls of the microfluidic channels that are hydrophilic or hydrophobic, as these can improve control of movement of fluids, for example by capillary effect. It is difficult to treat many materials to be hydrophilic, or hydrophobic. For materials that are amenable to such treatment, the effect may remain only for a given time, which leads to a short life for the microfluidic device. Furthermore, reliability of the treatment may be lacking: some chips may exhibit a lack of hydrophilicity or hydrophobicity having encountered similar storage regimens. Sometimes making the walls bond with the fluid results in interactions with the fluid that may contaminate, dilute, or otherwise alter the liquid. Furthermore, requiring that a liquid be hydrophilic or hydrophilic may have drawbacks for particular reactions or reagents.
Accordingly there have been many applications that use pneumatics to control movement of fluids through microfluidic chambers. These may roughly be divided into two groups: direct pneumatic control (e.g. WO 0177683 to Chow et al.), where the same microfluidic channels that transport fluids are in contact with pressurized gas, and pneumatic control layer that overlies a microfluidic chip, applying pressure to expand or contract the channels (e.g. Applicant's co-pending U.S. Ser. Nos. 12/588,236, 13/643,426, and 13/985,317). There are limits on how well fluid can be controlled in complex microfluidic chips with only controlling pressure at a fixed number of ports of a chip. A modestly complex process with a few liquid sources and reaction chambers typically requires a fairly large number of pressure supply lines that substantially increase a complexity of the equipment required for operation, resulting in a large network of pressure supply lines and attendant equipment.
One disadvantage of this technique is the cumbersome equipment that is required to operate a microfluidic chip. In the background of WO 2013/053039, Gray et al. notes that microfluidic pneumatic valving requires a large amount of support equipment to drive arrays of valves, resulting in a “chip-in-lab” situation rather than a self-contained “lab-on-chip”.
Centrifugal microfluidics is a branch of microfluidics that uses a centrifugal field to control movement of fluids within a microfluidic device: a microfluidic chip is mounted to a centrifuge. The centrifuge produces a centripetal field that varies continuously across the microfluidic chip, and draws the fluid to a lowest part of any chamber they are in (i.e. away from the center of rotation), or more generally, to whatever supporting wall is available (in the absence of which the fluid accelerates). The field has developed an array of chips, protocols, and tests, including lime staining assays (Chen, Li et al. 2010 “A rotating microfluidic array chip for staining assays.” Talanta 81(4-5): 1203-1208), whole cell sensing (Date, Pasini et al. 2010 “Integration of spore-based genetically engineered whole-cell sensing systems into portable centrifugal microfluidic platforms” Analytical and bioanalytical chemistry 398: 349-356.), real-time PCR (Jia, Ma et al. 2004), and single molecule detection (Melin, Johansson et al. 2005 “Thermoplastic microfluidic platform for single-molecule detection, cell culture, and actuation” Analytical chemistry 77(22): 7122-7130).
Spatial and temporal control of liquids in centrifugal microfluidic devices have been achieved by controlling in-plane structure of the microfluidic channels, as well as wetting properties of the materials used for fabricating the chips (Zoval and Madou 2004 “Centrifuge-based fluidic platforms.” Proceedings of the IEEE 92(1): 140-153; Lu, Juang et al. 2006 Superhydrophobic valve for microfluidics. Annual Technical Conference—ANTEC, Conference Proceedings, Charlotte, N.C.; Ducree, Haebrle et al. 2007 “The centrifugal microfluidic Bio-Disk platform.” Journal of Micromechanics and Microengineering 17(7)). Valving is achieved by capillary valves and siphon valves. Applicant has a co-pending application on metering and time control in centrifugal microfluidics (W02013/003935). Some centrifugal microfluidic devices have valves that are designed to control release of liquids at different places or at different rotation frequencies of the centrifuge. Thus there are many applications that can be provided using centrifugal microfluidics.
Nonetheless there are limitations on the existing methods of control in centrifugal microfluidics. As mentioned above, the use of surface treatments to control wetting has problems, and control of the liquid (by siphon valves or capillaries) is sensitive to wetting (contact angle of the liquid). There are important limits on what liquids will be valved. Furthermore there are limits to locations where capillary valves can be positioned because of their burst frequencies. Uncertainty of contact angle hysteresis is another issue. Accordingly a very small number (e.g. not more than 3) of capillary valves can operate at a required separation for the rotation speeds of typical centrifuges to avoid overlapping operations for typical device footprints. Timing may become an important constraint with some siphon valves, as siphon valves work by retarding flow, without discrete stops. In those siphon valves that can stop fluid flow permanently, a control over an angle of the chip with respect to the centrifugal field is required. Each valve provides an independent set of time constraints that depend on the liquid and surface treatment. This means that a duration of the effective valve limits how long other processes must be completed, which adds constraints to the design of the microfluidic process for which a chip is designed. Finally the geometry and surface treatments are both important features and it can be complicated to control capillary valves accurately, even for a narrow range of liquids, as patterning defects can cause additional variability.
In addition to these problems, there are other issues with control over liquids in centrifugal microfluidics, in that unidirectional flow is generally a problem, mixing is difficult to accomplish (although a good solution is taught by Applicant in WO 2013/0120190), and problems with loading and unloading liquids can require complex ancillary equipment.
US 2007/0059208 to Desmond teaches a rotating fluid processing device to move fluids introduced into input chambers radially outward through pathways. Desmond teaches introducing the fluids in a variety of ways, but these have to be performed prior to chip rotation.
U.S. Pat. No. 7,152,616 to Zucchelli et al. teaches, with regards to FIG. 6, use of a centrifugal microfluidic chip having air plugs to move liquid from an outer to an inner radial position, which is referred to as “reflow”. The reflow appears to be controlled by perforating a material layer when the centrifuge is not in operation. As the centrifuge controls fluid movement across the chip, and the operation of the reflow, it is impossible to independently control the movement of the fluids (away from the reflow area), and the reflow. The chip space required to enable limited reflow, and the constraints on channels in accordance with this technique are considerable disadvantages. Other similar approaches have been discussed, including Gorkin III, Clime et al. 2010 “Pneumatic pumping in centrifugal microfluidic platforms.” Microfluidics and Nanofluidics 9: 541-549, and thermal expansion of gasses in non-contact heated reservoirs (Abi-Samra, Clime et al. 2011 “Thermo-pneumatic pumping in centrifugal microfluidic platforms.” Microfluidics and Nanofluidics 11(643-652)).
Previously noted WO 2013/053039 to Gray et al., teaches a 2D array of microfluidic channels interconnecting wells with a number of electronically controlled valves at intersections between the channels, to produce a reconfigurable fluid routing track between the sample wells. As noted by Gray et al., “When scaled to microscale fluidic channels, surface tension, capillary forces, and other fluid dynamics become major considerations. Microfluidics applications usually require external pressure sources through pumps or centrifugal force; or electrokinetics for flow.” Gray et al. teach using magnetic pumping as the source for driving fluids.
Gray et al. notes an importance of separating contaminated elements of a microfluidic system, from reusable parts, and specifically defines cartridges for the microfluidics, that include the 2D array. The advantage of reconfigurability offered by this system may not warrant the costs of producing these cartridges. The cartridges require patterning and soft lithography, as well as impregnation with magnetic particles, and inlaid hydro-gel based microactuated valves. The production of these cartridges will therefore be expensive, and require many supplies and many electrical connections. The cartridges would be too expensive single-use items for many applications. Additionally, the hydrogel microvalves may be reactive with certain microfluids under assay. High voltages, and magnetic fields, are required for fluid control, and very slow movement of the microfluids are observed, leading to very slow processes, if the fluid must move from one chamber to another, or mix with reagents, etc.
Some prior art efforts have been made to combine the advantages of centrifugation with pneumatics. For example, Kong and Salin 2011 “A Valveless Pneumatic Fluid Transfer Technique Applied To Standard Additions on a Centrifugal Microfluidic Platform.” Analytical Chemistry 83(23): 9186-9190 teaches a pneumatic fluid transfer technique that uses compressed gas to generate a pneumatic force that works with the centrifugal force to direct fluid flow through a chip while the chip is under centrifugation. Kong and Salin teach blowing compressed air from a fixed external reservoir onto a surface of a rotating microfluidic chip. Liquid in the chip experiences a force pulse each time an access port on the surface passes across the stream blown by the external reservoir through a thin tube (nozzle). A solenoid valve is used for switching the pressure on and off, according to the paper. While this was demonstrated to be a useful technique for a relatively simple problem, it is not trivial to provide temporally accurate delivery of fluid in time with low cost pneumatic supplies, where the pressurized fluid supply is intermittently connected to the vent during rotation of the chip.
Accordingly, there is a need for a centrifugal microfluidic chip control that allows for more efficient microfluid control than centrifugal, or pneumatic techniques permit independently, and particularly to control that allows for processes to be performed with less user-intervention or avoid one or more identified limitations with the prior art.